This invention relates to an internal combustion engine and more particularly to a variable valve timing mechanism for operating the valves of such an engine.
It is well known that one of the factors that controls the performance of an internal combustion engine is the valve timing mechanism. Generally, the valves are operated by one or more camshafts at a timed relationship to the rotation of the engine output shaft. Frequently, the intake valves are operated by the camshaft that is different from that which operates the exhaust valves. Generally, the optical valve timing for an engine varies, depending upon the speed and load at which the engine is operating. Thus, conventional engines having fixed valve timing arrangement generally are designed to provide a comprise between good running at low speeds and low loads and maximum engine output. Depending upon the use of the engine, the bias may be toward one or the other end of these two alternative ranges.
In order to improve performance over a wider range of engine speed and load conditions, it has been proposed to employ a variable valve timing arrangement in the drive for the camshafts. In this way, the timing relationship of the camshafts can be adjusted so as to provide optimal performance for more running condition.
The variable valve timing mechanisms which have been proposed generally fall into two categories. With the first of these and the simpler arrangement, the timing of both camshafts is generally altered in the same direction and at the same degree. This is done by interposing one variable valve timing mechanism in the timing drive between the engine output shaft and the camshafts. This has the advantages of simplicity, lower cost and still provides greater flexibility in engine performance.
The other type of system includes a variable valve timing mechanism that is interposed between the drive for each of the camshafts from the output shaft. This obviously doubles the number of components, including the control mechanism. It does, however, offer the possibility of a greater flexibility in overall engine performance.
It has been discovered, however, that there are a number of running conditions where the performance is optimal if both camshafts are adjusted at the same phase angle. Other running conditions require a different adjustment between the camshafts. Although this can be employed were it easily in an arrangement wherein there are independent variable valve timing mechanisms associated with each camshaft, this makes the control strategy more complicated.
It is, therefore, a principal object of this invention to provide an improved variable valve timing mechanism for an engine wherein two camshafts may have their timing altered simultaneously or independently of each other, depending upon the running characteristics.
It is a further object of this invention to provide an improved camshaft variable valve timing mechanism wherein simultaneous adjustment of both camshafts can be accomplished through the use of one variable valve timing mechanism and adjustment of the timing of the camshafts relative to each other is done by a separate variable valve timing mechanism.
This invention is adapted to be embodied in an internal combustion engine and camshaft timing drive therefore. The engine comprises an output shaft that is driven by combustion occurring in at least one combustion chamber of the engine. A first camshaft operates at least one valve associated with the combustion chamber. A second camshaft operates at least a second valve also associated with the combustion chamber. First and second timing drives are provided for driving the camshafts at the same speed from the output shaft. A first variable valve timing arrangement is provided for adjusting the timing of both of the camshafts simultaneously and in the same degree and same direction relative to the rotation of the engine output shaft. A second variable valve timing mechanism is interposed in the timing drive for varying the timing between the two camshafts.
In accordance with the preferred embodiments of the invention that are illustrated, the first and second camshafts each drive either intake or exhaust camshafts. Although such an arrangement is possible, it also may be desirable to operate the valves of a multiple valve engine so that the timing of valves that serve the same function (either intake valves or exhaust valves), can have their timing adjusted either simultaneously or independently of each other. Thus, although the illustrated embodiments show an arrangement wherein the intake and exhaust valves are timed separately, the invention also may be practiced with engines where valves that serve the same function (either intake or exhaust or both) can be operated simultaneously or independently. The structure for accomplishing this should be readily apparent from the description of the preferred embodiments which are disclosed.